Documentation: add dsc and mst node information for sm8150

Add DSC resource capability nodes for sm8150 and also
add the missed documentation for mst and dsc feature enable nodes.

Change-Id: Ia0c919a1dc2014fbc5b3f0d94c663e28b06f57f4
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>